Evaluation of shielding analysis methods in spent-fuel cask environments

The three-dimensional Monte Carlo code MORSE-SGC, as implemented in the SCALE system calculational sequence SAS4, is applied to the analysis of a series of simple geometry benchmark experiments and prototypic spent-fuel storage cask measurements. The simple geometry experiments were performed in Japan and at the General Electric-Morris Operation facility; the cask measurements were performed at the Idaho National Engineering Laboratory. The quantification of uncertainties in a typical shielding analysis process for transport/storage casks can be accomplished by comparison of consistent trends between calculated and measured dose rate quantities in both benchmark and prototypic environments. Benchmark results typically measure the validity of cross-section data and computer code adequacy; prototypic environments, however, generally measure the overall validity of the calculational procedure. A total of five storage cask problems and two simple geometry problems were analyzed to determine the expected accuracies of computational analyses using well-established source-generation and Monte Carlo codes. The general trends seen in this work are in agreement within 30% or better with the measurements for neutron dose rates along the cask side, lid, and bottom. The gamma-ray dose rates with substantial contributions from the top endfitting, plenum, and bottom endfitting regions also are in good agreement. Based on the latest results, gamma-ray dose rate calculations with major contributions due to the active fuel region show a consistent factor of 1.6 overprediction of the measured quantities for casks with iron and concrete shields. Major uncertainties exist in the quantification of Co-59 concentrations in endfitting hardware materials. The results presented support the accuracy of source generation methods and dose estimation methods in these regions given accurate impurity characterizations. Thus, it is felt that the practice of using upper bounds for Co-59 initial concentrations should ensure conservative cask designs. Fortunately, the gamma-ray dose discrepancies seen along the sides of both the iron and concrete cask surfaces are overpredictions. The reason for overprediction is not fully known. Even though these overpredictions are not clearly understood, the trends observed, combined with some degree of code and data testing using these or similar benchmark measurements, should inspire confidence in the shielding results for a shipping/storage package.